1. Field of the Invention
The present invention relates to a method of stripping a remnant metal, and more particularly, to a method that produces a protective layer on a transitional silicide and performs a stripping process to strip a remnant metal on the transitional silicide.
2. Description of the Prior Art
Transistors are important electrical components utilized in the manufacture of integrated circuits. As semiconductors become smaller and smaller, the manufacture of transistors has undergone great improvement in order to produce transistors of small size and high quality.
In current transistor manufacturing, manufacture of silicide is a method of improving transistor quality. The common method of manufacturing silicide involves self-aligned silicide. The method includes covering a metal layer (e.g. nickel, Ni) on the source/drain and the gate after the source/drain has been manufactured. A titanium nitride (TiN) layer is sputtered on the metal layer. Next, a rapid thermal process (RTP) is performed, and the metal in the metal layer reacts with the silicon in the gate and the source/drain to form transitional silicide on the surfaces of the gate and the source/drain. Afterward, the un-reacted metal layer is removed, and another RTP is processed to transform the transitional silicide on the surfaces of the gate and the source/drain to silicide having lower resistance.
In general, the advantage of forming silicide on the surfaces of the gate and the source/drain is that the Ohmic contact between the gate, the source/drain and the contact plugs is improved. The materials of the contact plugs are conductive metal, such as tungsten (W). The polycrystalline silicon or monocrystalline silicon of the gate and the source/drain have a bad electrical conduction with the contact plugs. Therefore, the silicide is formed on the gate and the source/drain to improve the Ohmic contact between the gate, the source/drain and the conduct plugs, and to enable the sheet resistances of the source/drain to decrease at the same time.
To prevent the silicide (e.g. nickel silicide, NiSi) from agglomerating, the metal layer, which is utilized to form the silicide, includes a low concentration stable metal. The agglomeration in the silicide increases the contact resistance of the contact plug so junction leakage occurs. Therefore, thermally stable metal is added in the metal layer to prevent the agglomeration. For example, 3-8 wt % of platinum (Pt) is added in the Ni metal layer, which is utilized to form the silicide. Because Pt has a stable chemical property, Pt improves the thermal stability of NiSi. So, the NiSi will not agglomerate at a higher temperature after Pt is added.
The strip selectivity between Pt and NiSi is small. When the stripping process is performed to strip the un-reacted Pt, the formed NiSi is stripped at the same time.
Please refer to FIGS. 1-2. FIGS. 1-2 are schematic diagrams of manufacturing the silicide in the prior art. As FIG. 1 shows, a semiconductor wafer 10 has a substrate 12 (e.g. a silicon substrate). A gate 20 is formed on the substrate 12, and the gate 20 includes a gate insulating layer 14 and a gate conductive layer 16. Next, an ion implantation, which decreases the thermal budget, is performed, and source/drain extensions 26 (also called lightly doped drains, LDD) are formed in the substrate 12 of the two lateral sides of the gate 20. Next, a spacer 18 is formed around the gate 20. The gate 20 and the spacer 18 are utilized as masks and an ion implantation is processed to form the source/drain 28 in the substrate 12.
Next, a thin film deposition process is performed, and a metal layer 22 is uniformly formed on the substrate 12 and the gate 22. The metal layer 22 includes 3-8 wt % of Pt and 92-97% Ni. Then, a TiN layer 24 is sputtered on the metal layer 22.
As FIG. 2 shows, a first RTP is performed, and the partial metal layer 22 reacts with the gate conductive layer 16 under the metal layer 22, and the silicon in the source/drain 28 to form a transitional silicide 30. The reaction of the first RTP is:Si+Ni->Ni2Si
Next, a sulfuric acid-hydrogen peroxide mixture (SPM) striping process is performed, and the TiN layer 24 and the un-reacted Ni in the metal layer 22 are removed. A hydrochloric acid hydrogen peroxide mixture (HPM) is utilized, which reacts with un-reacted Pt above the transitional silicide 30 to generate the complex ions, thereby stripping the un-reacted Pt.
The HPM includes hydrogen peroxide, hydrochloric acid, and chlorine (Cl2). These materials damage the transitional silicide 30; for example, Cl2 is a highly reactive gas, and reacts with metal. As well as reacting with Pt, Cl2 also reacts with the transitional silicide. The transitional silicide is etched and is stripped. The second RTP is performed, and the transitional silicide 30 becomes silicide having a lower resistance. The reaction of the second RTP is:Si+NiSi->NiSi
Please refer to FIG. 3. FIG. 3 is a scanning electron microscopy (SEM) diagram of the un-reacted Pt on the transitional silicide of FIG. 2. As FIG. 3 shows, the substrate 12 has a plurality of transitional silicide 30. The un-reacted Pt 32 lies above the partial transitional silicide 30. Please refer to FIG. 4. FIG. 4 is a SEM diagram of the un-reacted Pt FIG. 2 after the HPM stripping process. As FIG. 4 shows, the HPM striping process can strip the un-reacted Pt, and the transitional silicide 30 on the substrate 12 reacts with Cl2 of HPM. The partial transitional silicide 30 is etched and is stripped to form the damaged region 42.